How long will this keep?

That sort of depends on what kind of willpower you possess, and how big you cut each portion. For long term storage, it would be best to freeze it. From personal experience, cut,wrapped and frozen individual portions lasted just shy of three months. In the fridge, it never lasts more than a couple days because the entire family sneeks slivers and bits until it is all gone. If you had excellent self control, I imagine it could last a week safely.

Well covered, quite a while. Took me five days to finish the one I made, and it was still fine- it's practically all butter and chocolate, after all, not much is going to happen to it.